Detecting Heterobasidion irregulare in Minnesota and Assessment of Indigenous Fungi on Pines

Eric Otto,Samuel Redford,Benjamin Held,Robert A Blanchette

doi:10.3390/f12010057

Abstract

Heterobasidion irregulare is one of the most problematic forest pathogens in the northern hemisphere, but has only been found relatively recently in the north central United States. Discovered in Wisconsin in 1993, but probably established sometime before that, it quickly spread throughout the state. In November 2014, it was found in southeastern Minnesota. Field surveys were then conducted throughout Minnesota with the focus in the southeast near the initial discovery. To find additional infection sites, surveys were conducted with accompanying aerial imagery of red pine (Pinus resinosa Aiton) stands that were previously thinned. Samples were collected from selected sites with dead and dying trees as well as samples from stumps in recently thinned pine stands. These samples were processed first with a nested polymerase chain reaction (PCR) protocol, which was replaced by a real-time PCR assay after its development. No samples tested positive for H. irregulare using these methods and no cultures from isolations were obtained outside the original infection area. Other indigenous fungi were also identified. The majority were wood decay fungi in the Basidiomycota. A spore collection study was also conducted after field surveys. Automated rotary arm spore collectors were used and assayed with an ITS TaqMan real-time PCR assay. Collectors were placed strategically in different areas of Minnesota. A positive control was used in an infected red pine plantation in Wisconsin and this location had the highest number of spores trapped, with 63,776 over a week period. Spores of H. irregulare were detected at several sites in Minnesota, with the highest spore total observed in traps at 413 over a week period. All other locations sampled also had some spores collected except Itasca State Park located in northwestern Minnesota. The weekly deposition of spores ranged from 0 to 1.26 m−2 h−1. Low spore levels occurring in Minnesota indicate that some spores are present, but they are currently being detected in amounts that may not be sufficient for colonization to be successful.

Highlights

The fungal pathogen, Heterobasidion irregulare Garbelotto & Otrosina, is a serious threat to red pine (Pinus resinosa Aiton) and other conifers throughout North America [1]
In addition to causing serious economic damage, Heterobasidion root disease (HRD) impacts ecosystem services and can be devastating in urban environments where trees weakened by infection can pose hazards to the general public
No additional basidiocarps were located in the field and no isolates of H. irregulare were obtained from woody samples that were cultured from suspect trees, stumps, and downed conifer wood

Summary

Introduction

The fungal pathogen, Heterobasidion irregulare Garbelotto & Otrosina, is a serious threat to red pine (Pinus resinosa Aiton) and other conifers throughout North America [1]. It is considered one of the most economically damaging forest pathogens in the northern temperate regions of the world. The primary infection is by airborne basidiospores produced by shelf-like basidiocarps [2]. Basidiospores can be disseminated overland to start new infections on recently cut stumps and wounds. The asexual state, Spiniger meineckellus (A.J. Olson) Staplers, produces large numbers of conidiophores and conidia that can form over the surface of exposed infected wood. The role of conidia for infection, is not well understood, 4.0/)

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Discussion

Conclusion